1. Field of the Invention
The present invention generally relates to radio communication, and especially relates to a communication device and method used under a frequency sharing environment where plural communication systems establish communications within the same frequency band.
2. Description of the Related Art
In the prior art radio communication systems, a dedicated frequency band is allocated to each radio communication system so as to avoid interference and maintain signal qualities. However, in order to use frequency resources much more efficiently, a frequency sharing environment is considered, in which plural communication systems share the same frequency bands. In such a frequency sharing environment, it is required that own system suppresses interfering signals from other systems to maintain the signal quality of desired signals for the own system.
FIG. 1 shows an example of two transmitters and a receiver that would be used in such a system. In the example shown in FIG. 1, two transmitters use the same frequency band, but a user 1 and a user 2 utilize different communication systems. A signal transmitted from the user 2 becomes an interfering signal against the user 1. In the prior art radio communication system, a transmission shaping filter 1 in the transmitter for the user 1 and a receiver shaping filter 3 in a receiver for the user 1 make a pair and their filter transfer characteristics are fixed so as to perform suitable band pass limitation.
FIG. 2 shows frequency spectrum charts illustrating a desired signal. A1 represents a baseband signal (frequency spectrum) of impulse series of a modulated signal for user 1. B1 represents a baseband signal frequency spectrum of user 1 that has been bandpass-filtered by a transmission shaping filter (e.g., a root-raised cosine filter). C1 represents a frequency spectrum of an RF transmission signal transmitted from the user 1. The desired signal transmitted by user 1 has a carrier frequency of f1. The desired signal is transmitted with a symbol interval of T1, and thus its Nyquist frequency is ½T1.
FIG. 3 shows frequency spectrum charts illustrating an undesired (interfering) signal. A2 represents a baseband signal (frequency spectrum) of impulse series of a modulated signal for user 2. B2 represents a baseband signal frequency spectrum of user 2 that has been bandpass-filtered by a transmission shaping filter (e.g., a root-raised cosine filter). C2 represents a frequency spectrum of an RF transmission signal transmitted from the user 2. The undesired (interfering) signal transmitted by user 2 has a carrier frequency of f2. The undesired signal is transmitted with a symbol interval of T2, and thus its Nyquist frequency is ½T2.
FIG. 4 shows frequency charts illustrating a desired signal. D represents a frequency spectrum of signals received by the receiver for the user 1. The total spectrum (solid line) of the received signal is considered to consist of a desired signal (1), an undesired signal (2) and noise (3). E represents a frequency spectrum of signals converted from RF to baseband. F represents a frequency spectrum of signals after bandpass filtering by a receiver shaping filter 3 in the receiver for the user 1.
FIG. 5 shows frequency charts illustrating a desired signal that has been ideally equalized. G represents the frequency spectrum of a desired signal after being ideally equalized by an adaptive filter in the receiver for the user 1. The signal shown in FIG. 5G is symbol-rate-sampled to repeatedly appear with intervals of 1/T1 in the frequency domain as shown in FIG. 5H. The summation of these frequency spectrum signals yields a restored signal (FIG. 5I) that corresponds to the transmitted signal from the user 1. The signals indicated by A1, B1, C1, . . . in FIGS. 2-5 correspond to signals indicated by the same reference characters in FIG. 1.
One method for cancelling another system's interfering signals in received signals is known, in which Maximum Likelihood Sequence or Linear signal processing is employed to jointly or sequentially process the signals, respectively. These methods, however, require that the desired system grasps the parameters (training symbol, modulation method, symbol rate, etc.) of the interfering system in advance. When such parameters of the interfering system are unknown, it is impossible to effectively cancel the interfering signals.
Another method for cancelling another system's interfering signals in received signals is known, in which the Fractionally Spaced Equalizer (FSE) and the FREquency Shift filtering (FRESH) are employed. FSE and FRESH filtering schemes have the potential to exploit the spectrum redundancy for interference cancellation with no need for interfering system signal's parameters. FIG. 6 schematically shows a frequency shift filter (FRESH). FIG. 7 schematically shows a fractionally spaced equalizer (FSE). The FSE and FRESH can be used as an adaptive filter for the receiver as shown in FIG. 1. As shown in FIG. 6, the FRESH includes a plurality of FSEs connected in parallel, and the outputs of the FSEs are summed. The output after the summation is subtracted from a training signal to generate an error signal. The tap coefficients of all FSE filters are jointly adjusted so as to reduce the error signal. As shown in FIG. 7, the FSE filter includes a series of delay elements, each of which delays an over-sampled input signal. Each output from each of the delay elements is multiplied with a tap coefficient (i.e., weight) ci and summed (see Non-Patent Documents #1, #2).
[Non-Patent Document #1]
W. A. Gardner, “Exploitation of spectral redundancy” in cyclostationary signals”, IEEE Signal Processing Magazine, vol. 8, no. 2, pp. 14-36, April 1991
[Non-Patent Document #2]
W. A. Gardner, “Cyclic Wiener filtering: theory and method”, IEEE Trans. Commun., vol. 41, no. 1, pp. 151-163, January 1993